The characterization of a particle or an ensemble of particles represents an important determination for many industries. For example, in the beverage industry, an objective means by which a particular drink is characterized plays a major role in quality control. Many chemical processes require a controlled particle size distribution and density prior to the initiation of various chemical reactions. This in turn requires some relatively simple means for identifying those size parameters and particle density properties before their introduction into the process itself. Many measurements in the food industry relating to quality control actually have their origin in subjective taste testing. A suitable characterization of the particles giving rise to the particular tastes of importance would result in a simpler and more reproducible test.
The measurement and interpretation of the response of a microparticle to its environment is an important objective of numerous determinations. In the area of immunochemistry, for example, the concentration of antibodies in a patient's serum is generally determined by exposing such antibodies to an environment of antigens specific to them or complexes created from them and monitoring their reaction. For antibiotic chemotherapy studies, bacterial isolates are often exposed to an antibiotic environment to determine the potential effectiveness of such drugs. Effects of temperature and various chemical concentrations on chemical reactions invariably involve similar measurement techniques.
The presence of adulterants and/or toxicants in food and water supplies are generally detected by means of particles, including molecules, which undergo significant physical changes in their environment. There are at present several methods by which such environmentally-caused changes may be detected and monitored and particles themselves characterized.
Turbidimetric and nephelometric techniques are most prevelant. Such measurements are often used to characterize particle density. Changes in light transmission or the amount of light scattered in a particular direction forms the basis, respectively, of these two methods. Although attractive for their instrumental simplicity, these methods are not particularly sensitive nor accurate because of inherent experimental problems including, but not limited to, high background fluctuations and contributions, light source instabilities, and lack of suitable reference standards.
A method of considerable promise for characterizing particles or measuring their environmental response is that of differential light scattering (DLS) whereby detailed variations of light scattered intensity with angle are measured and recorded. The resulting DLS patterns are then compared to yield a measurement of particle morphological change, as well as number density variations. Comparing with a reference standard can result in a useful particle characterization procedure. The DLS technique requires extensive detection electronics, as well as complex computer interpretation routines. The DLS approach also depends critically upon the shape of the recorded light scattering pattern and its results are very susceptible to small shape changes that may arise from artifacts such as debris or markings/irregularities of the sample holder.
Electrical impedance methods are often used to determine particle size distributions and number densities. Such measurements performed before and after exposure to various environments provide a means for monitoring their response as well as yielding important characterization properties. Particles are suspended in a saline solution and forced through a fine capillary tube across which an electrical potential is applied. As each particle traverses the capillary an impedance change occurs due to the particle physically obstructing the electrical conduction path. The resultant pulse is said to be proportional to the particle's physical cross section. The basic shortcomings of the method include capillary plugging, requiring frequent cleaning, and lack of adequate size resolution below a few micrometers.
Radiometric measurements are of frequent use, especially for immunochemical determinations, yet despite their inherent accuracy, these techniques invariably present the user with problems of waste disposal and health hazards. Numerous other measurement techniques exist for characterizing particles and monitoring their environmental response, including colormetric reactions, plate diffusion methods, electrochemical impedance changes, and a variety of precipitation techniques.